CN112015220A

CN112015220A - Current limiter

Info

Publication number: CN112015220A
Application number: CN201910458075.7A
Authority: CN
Inventors: 杨曜玮
Original assignee: Elite Semiconductor Memory Technology Inc
Current assignee: Elite Semiconductor Memory Technology Inc
Priority date: 2019-05-29
Filing date: 2019-05-29
Publication date: 2020-12-01
Anticipated expiration: 2039-05-29
Also published as: CN112015220B

Abstract

The invention provides a current limiter, which is used for a voltage regulator and comprises a sensing current source, a current limiting regulation unit, a variable current source, an operational amplifier and a control switch. The sensing current source is electrically connected to the voltage regulator. The current limiting adjusting unit is electrically connected to the voltage regulator. The variable current source is connected in parallel with the sensing current source and is electrically connected with the current limiting regulation unit. The operational amplifier is electrically connected to the sensing current source. The control switch is electrically connected to the voltage regulator and the operational amplifier. When the divided voltage of the output voltage of the voltage regulator gradually decreases to be lower than a certain value, the variable current source gradually adjusts and increases the current generated by the variable current source according to the first control signal.

Description

Current limiter

Technical Field

The present invention relates to a current limiter, and more particularly, to a current limiter for a voltage regulator.

Background

A prior art voltage regulator, particularly a Linear regulator (Linear regulator), regulates the current flowing through a load with a transistor (or other device), and the voltage obtained by the load is the output voltage of the voltage regulator. The output voltage is compared with the reference voltage in the voltage regulator, the generated differential signal is used as a control transistor to form a negative feedback loop, and the output voltage can be adjusted and reduced to the target voltage by adding proper compensation, so that the output voltage is not influenced by the input voltage or load change, and the reasonable stable output voltage is kept.

However, the linear regulator must output the input voltage higher than the output voltage by no less than a certain voltage value before the output voltage is stabilized to the target voltage, and this minimum voltage is called a droop voltage, or a voltage difference (droop). For example, if the output is to be kept at 5V, the input must be kept above 7V, otherwise the output will be below the target voltage of 5V, where the drop voltage is 7V-5V-2V, and the energy generated by the drop voltage is dissipated in the transistor as heat energy.

Referring to fig. 1, fig. 1 is a schematic diagram illustrating an operation relationship between a voltage regulator and a current limiter in the prior art. As shown in fig. 1, the voltage regulator 10 is electrically connected to the current limiter 20, and includes an operational amplifier 11, a transistor 12 (typically a PMOS transistor), a first voltage dividing resistor 13 and a second voltage dividing resistor 14. The source of the PMOS transistor 12 is electrically coupled to the input voltage V_INThe drain of the PMOS transistor 12 is connected to one end of the second voltage-dividing resistor 14. The other end of the second voltage-dividing resistor 14 is connected to one end of the first voltage-dividing resistor 13, and the other end of the first voltage-dividing resistor 13 is electrically coupled to ground. The operational amplifier 11 has an output terminal, an inverting input terminal, and a non-inverting input terminal. The inverting input terminal of the operational amplifier 11 is electrically coupled to the reference voltage V_REFThe output terminal of the operational amplifier 11 is electrically coupled to the gate of the transistor 12. The other end of the first voltage-dividing resistor 13 and one end of the second voltage-dividing resistor 14 are connected to the non-inverting input terminal of the operational amplifier 11. The current limiter 20 is electrically coupled to the operational amplifier 11 and the input voltage V_INBetween

In FIG. 1, the current I is input_INFlows into the source of PMOS transistor 12, and flows out of the drain of PMOS transistor 12_OUTAnd feedback current I_FBSo that the input current I can be known_INOutput current I_OUT+ feedback current I_FB. In addition, the feedback current I_FBA voltage drop is generated by the second voltage dividing resistor 14, so that the output voltage V of the drain of the PMOS transistor 12_OUTA voltage drop is generated at the second voltage-dividing resistor 14, and a voltage V is generated at the other end of the second voltage-dividing resistor 14_FBThe voltage value of (2). The operational amplifier 11 is for maintaining V_FBSet V_REFVoltage, if the load becomes smaller, at output voltage V_OUTConstant input current I_INIs adjusted to be larger, and the input current I is increased_INTo a certain extent, this limitThe current transformer 20 is activated to input the current I_INIs limited to a fixed clamp current level to avoid damage to transistor 12 due to excessive current.

Referring to fig. 2A and 2B, fig. 2A shows the output voltage V of fig. 1 when the output voltage is lower than the set stable voltage_OUTAnd an output current I_OUTFIG. 2B is a graph showing the relationship between the output voltage V and the set stable voltage of FIG. 1_OUTAnd transistor power consumption P_LOSSThe relationship between them. At an input voltage V_INWhen fixed, when the output voltage V is_OUTBecomes smaller, and in the most extreme case, also at the output voltage V_OUTWhen the current becomes small enough to almost short the load, the maximum power consumption will be on the PMOS transistor 12, and as can be seen from fig. 2A, the current limiter 20 is controlled to the input current I_INThe maximum value of (a) is clamped at a certain fixed size. Due to power consumption P of the transistor_LOSSWill be equal to the input current I_INMultiplied by the input voltage V_INAnd an output voltage V_OUTThe difference between them, as can be seen from FIG. 2B, although the current I is outputted_INIs controlled at a maximum value I_{OUT_CL}(in general I)_{OUT_CL}Will be much larger than I_FBTherefore I is_INWill approximate I_{OUT_CL}) But when the output voltage V is_OUTWhen reduced, input voltage V_INAnd an output voltage V_OUTThe difference between them still continues to cause power consumption P of the PMOS transistor 12_LOSSAnd output a voltage V_OUTApproaching zero, there is maximum power consumption P_{LOSS_MAX}。

Disclosure of Invention

In view of the above-mentioned prior art, the inventor developed a current limiter for a voltage regulator, which can reduce the drop voltage by regulating the current through the current limiter of the present invention, thereby reducing the power loss caused by the transistor and achieving the purpose of protecting the voltage regulator from being damaged by overheating.

To achieve the above and other objects, the present invention provides a current limiter for a voltage regulator, including a sensing current source, a current limiting regulation unit, a variable current source, an operational amplifier, and a control switch. The sensing current source is electrically connected to the voltage regulator and is used for generating an induction current according to the input current of the voltage regulator. The current limiting and adjusting unit is electrically connected to the voltage regulator and is used for generating a first control signal according to the output voltage of the voltage regulator and a first reference voltage. The variable current source is connected in parallel with the sensing current source, is electrically connected with the current limiting and adjusting unit, and is used for adjusting the magnitude of the generated current according to the first control signal. The operational amplifier is electrically connected to the sensing current source and is used for generating a second control signal according to a sensing voltage generated by the sensing current source and the current generated by the variable current source and a second reference voltage. The control switch is electrically connected to the voltage regulator and the operational amplifier, and provides a second control signal according to the regulated result to a grid electrode of a transistor of the control switch. When the divided voltage of the output voltage of the voltage regulator gradually decreases, the variable current source gradually adjusts and increases the magnitude of the current generated by the variable current source according to the first control signal.

In one embodiment, the sensing current source has an input terminal and an output terminal, and the input terminal is electrically connected to the input voltage of the voltage regulator. The current limiter further comprises a resistor, and the output end of the sensing current source is grounded through the resistor. The magnitude of the output current of the sensing current source is proportional to the input current of the voltage regulator, and the voltage across the resistor is the output voltage of the sensing current source and the variable current source.

In an embodiment, the current-limiting adjusting unit has a first input terminal, a second input terminal and an output terminal, the first input terminal is grounded through a first voltage-dividing resistor of the voltage regulator, the second input terminal is connected to a first reference voltage, and the output terminal is electrically connected to the variable current source and outputs a first control signal to the variable current source through the output terminal.

In one embodiment, the variable current source has a first input terminal, a second input terminal and an output terminal, the first input terminal is electrically connected to the input terminal of the sensing current source, the output terminal is electrically connected to the output terminal of the sensing current source, and the second input terminal is electrically connected to the current limiting adjustment unit.

In one embodiment, the operational amplifier has a negative phase input terminal electrically connected to the output terminal of the sensing current source, a positive phase input terminal electrically connected to the second reference voltage, and an output terminal electrically connected to the control switch, and outputs the second control signal to the control switch through the output terminal.

In one embodiment, the control switch has a first input terminal electrically connected to the input voltage of the voltage regulator, a second input terminal electrically connected to the operational amplifier, and an output terminal connected to the gate of the transistor of the voltage regulator.

In one embodiment, the control switch includes a transistor, a first input terminal of the control switch is a source of the transistor, a second input terminal of the control switch is a gate of the transistor, and an output terminal of the control switch is a drain of the transistor.

In one embodiment, the sensing current source is a current mirror or a current amplifier.

In one embodiment, the current limiting adjustment unit is a differential amplifier.

The current limiter can adjust and limit the maximum value of the input current through the current limiting adjusting unit, thereby further reducing the power consumption of the transistor and effectively protecting the transistor from being damaged.

Drawings

FIG. 1 depicts a schematic diagram of the action relationship between a prior art voltage regulator and a current limiter;

FIG. 2A depicts a graph of output voltage versus output current for the graph of FIG. 1 when the output voltage is below a set regulated voltage;

FIG. 2B depicts a graph of output voltage versus transistor power consumption for the voltage of FIG. 1 when the output voltage is below a set regulated voltage;

FIG. 3 depicts a circuit diagram of an embodiment of the current limiter of the present invention;

FIG. 4 depicts a circuit diagram of an embodiment of a current limit adjustment unit;

FIG. 5 depicts a signal simulation diagram of an embodiment of the current limiter of the present invention;

FIG. 6A depicts a graph of output voltage versus output current of FIG. 3 when the current limiting mechanism is activated;

fig. 6B depicts a graph of the output voltage versus transistor power consumption of fig. 3 when the current limiting mechanism is activated.

[ List of reference numerals ]

10 voltage regulator

11 operational amplifier

12 PMOS transistor

13. 14 voltage dividing resistor

20. 40 flow restrictor

41 sense current source

42 current limiting regulating unit

43 variable current source

44 operational amplifier

45 control switch

46 resistance

DC current source

I_SENInduced current

I_MODVariable current

I_INInput current

I_OUT output current

I_FBFeedback current

M1, M2, M3 and M4 transistors

S _ C1, S _ C2 control signals

V_REF、V_MREF、V_OCREFReference voltage

V_INInput voltage

V_OUTOutput voltage

V_FBPartial pressure

V_SENInduced voltage

Detailed Description

For a fuller understanding of the objects, features and advantages of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:

referring to fig. 3, fig. 3 is a circuit diagram of a current limiter according to an embodiment of the present invention. The current limiter 40 of the present invention is used in a voltage regulator 10 and is adjustable to limit the output current I of the voltage regulator 10_OUTTo reduce power consumption of the PMOS transistor 12 in the voltage regulator 10 and to protect the transistor 12 of the voltage regulator 10 from damage. It should be noted that the voltage regulator 10 in fig. 3 and the voltage regulator 10 in fig. 1 have the same circuit structure and operation principle, and therefore the same reference numerals are used.

The current limiter 40 includes a sensing current source 41, a current limit adjusting unit 42, a variable current source 43, an operational amplifier 44, a control switch 45, and a resistor 46. The sensing current source 41 is electrically connected to the voltage regulator 10 and is based on the input current I of the voltage regulator 10_INTo generate an induced current I_SEN. The current limiting adjustment unit 42 is electrically connected to the voltage regulator 10 and is configured to adjust the output voltage V of the voltage regulator 10_OUTAnd a first reference voltage V_MREFTo generate the first control signal S _ C1. The variable current source 43 is connected in parallel to the sensing current source 41, electrically connected to the current limiting adjustment unit 42, and adjusts the generated current I according to the first control signal S _ C1_MODThe size of (2). The operational amplifier 44 is electrically connected to the sensing current source 41 and generates a voltage V with the resistor 46 according to the outputs of the sensing current source 41 and the variable current source 43_SENAnd is connected to a second reference voltage V_OCREFTo generate the second control signal S _ C2. The control switch 45 is electrically connected to the voltage regulator 10 and the operational amplifier 44, and adjusts the magnitude of the on-current flowing through the control switch 45 according to the second control signal S _ C2, so as to pull up the gate of the PMOS transistor 12 when the control switch 45 is turned on. When the output voltage V of the voltage regulator 10_OUTPartial pressure V of_FBWhen gradually generating, the variable current source 43 adjusts and reduces the generated current I according to the first control signal S _ C1_MODTo zero.

In an embodiment, the input terminal of the sensing current source 41 is electrically connected to the input voltage V_INThe output of the sensing current source 41 is connected to ground through a resistor 46. Please note that the current I outputted by the current source 41 is sensed_SENWill follow the input current I_INWhen the magnitude of the input current I varies_INWhen the current I outputted from the current source 41 increases, the current I is sensed_SENWill also become larger when the input current I is larger_INSensing the current I output by the current source 41 when it becomes small_SENAlso, the current I outputted from the current source 41 is reduced, i.e., sensed_SENIs related to the input current I_INProportional, i.e. the current I output by the sensing current source 41 can be sensed_SENViewed as the input current I of the voltage regulator 10_INExpressed by the mathematical formula (1) is:

I_SEN＝β*I_IN (1)

in addition, when the current I outputted by the current source 41 is sensed_SENAnd the output current I of the variable current source 43_MODWhen flowing through the resistor 46, a voltage V is generated across the resistor 46_SENWhen V is_SENClose to V_OCREFThe control switch 45 is activated.

In an embodiment, the sensing current source 41 may be a current mirror or a current amplifier, but the present invention is not limited thereto, and those skilled in the art can understand the operation principle of the sensing current source 41 after referring to the above paragraphs, and perform equivalent changes and substitutions according to actual requirements, so that the output current I of the sensing current source 41 can be changed_SENAnd the input current I of the voltage regulator_INThe embodiments with proportional relationship are all within the scope of the present invention.

In one embodiment, a first input terminal of the current limiting and adjusting unit 42 is connected to ground through the first voltage dividing resistor 13 of the voltage regulator 10, and a second input terminal of the current limiting and adjusting unit 42 is connected to the first reference voltage V_MREFThe output terminal of the current limiting adjustment unit 42 is electrically connected to the variable current source 43, so as to send the first control signal S _ C1 to the variable current source 43. The current limiting regulation unit 42 receives the output voltage V from the voltage regulator 10_OUTThe partial voltage V generated to the first voltage dividing resistor 13_FBAnd a first reference voltage V_MREFAnd generates a corresponding first control signal S _ C1 accordingly. In practice, the current limiting and adjusting unit 42 is based on the divided voltage VFB and the first reference voltage V_MREFThe difference between the first and second signals determines the magnitude of the first control signal S _ C1, and the variable current source 43 receives the first control signal S _ C1 and determines the current I according to the first control signal S _ C1_MODThe size of (2). When current limiting protection occurs, and in a steady state when the current limiting regulation unit 42 is activated, the voltage V is sensed_SENIs fixed, so that the current I is induced_SENAnd the current I generated by the variable current source 43_MODIs fixed, so that when the current I generated by the variable current source 43 is constant_MODWhen the current is adjusted to be larger, the current I is induced_SENIt will be smaller.

In an embodiment, the current limiting adjusting unit 42 may be a differential amplifier driven by a current source, and the variable current source 43 may be a current mirror, but the present invention is not limited thereto, and a person skilled in the art can understand the operation principle between the current limiting adjusting unit 42 and the variable current source 43 after referring to the above paragraphs, and perform equivalent variation and replacement according to the actual requirement, so that the current limiting adjusting unit 42 can be enabled to divide the voltage V_FBWhen decreasing, the variable current source 43 is controlled to regulate the output current I_MODThe present invention is intended to cover various embodiments, all of which are within the scope of the present invention.

For example, referring to fig. 4, fig. 4 is a circuit diagram of an embodiment of a current limiting adjustment unit. In the present embodiment, the current limiting adjustment unit 42 includes a current source DC, a first transistor M1, a second transistor M2, a third transistor M3, and a fourth transistor M4. The output terminal of the current source DC is connected to the source of the first transistor M1 and the source of the second transistor M2, respectively, the drain of the first transistor M1 is connected in series to the drain of the third transistor M3, the drain of the second transistor M2 is connected in series to the drain of the fourth transistor M4, the gate of the third transistor M3 is connected to the drain of the third transistor M3, the gate of the fourth transistor M4 is connected to the drain of the fourth transistor M4, and the source of the third transistor M3 is connected to the source of the fourth transistor M4. A first reference voltage V_MREFIs connected toThe gate of the second transistor M2 is used to control whether the second transistor M2 is turned on or not, and the divided voltage V is_FBThe gate of the first transistor M1 is connected to control whether the first transistor M1 is turned on or not, and the gate of the third transistor M3 is connected to the variable current source 43. In other words, the first input terminal of the current-limiting adjusting unit 42 is the gate of the second transistor M2, the second input terminal of the current-limiting adjusting unit 42 is the gate of the first transistor M1, and the output terminal of the current-limiting adjusting unit 42 is the gate and the drain of the third transistor M3. The current from the current source DC is divided by the voltage V_FBAnd a first reference voltage V_MREFDivides the current through transistors M1, M3 and the current through transistors M2, M4. When the current flowing through the transistor M3 changes, the gate and drain voltages of the transistor M3 also change, so that the variable current source 43 receives the first control signal S _ C1 accordingly.

In one embodiment, the negative input terminal of the operational amplifier 44 is electrically connected to the output terminals of the sensing current source 41 and the variable current source 43, and the positive input terminal of the operational amplifier 44 is connected to the second reference voltage V_OCREFThe output terminal of the operational amplifier 44 is electrically connected to the control switch 45, so as to output the second control signal S _ C2 to the control switch 45. The operational amplifier 44 will sense the output voltage V of the current source 41 and the variable current source 43_SENMaximum value of (V)_{SEN_MAX}Limited to a second reference voltage V_OCREFIf the formula (1) is combined, the following results are obtained:

V_OCREF＝V_{SEN_MAX}＝(β*I_{OUT_CL}+I_MOD) Resistance value (2) of x resistor 46

Note that the output current I due to the variable current source 43_MODAnd the output current I of the sensing current source 41_SENThe sum of the current values is a constant value, so that when the output current I of the variable current source 43 is equal to_MODWhen the current becomes larger, the output current I of the current source 41 is sensed_SENThe input current I of the voltage regulator 10 is correspondingly decreased according to the formula (2)_INMaximum value of (I)_{OUT_CL}) And therefore, the power consumption of the voltage regulator 10 can be reduced, and the transistor 12 can be protected from damage.

The control switch 45 has a first input end electrically connected to the input voltage V of the voltage regulator 10, a second input end and an output end_INThe second input terminal is electrically connected to the output terminal of the operational amplifier 44, and the output terminal of the control switch is connected to the gate of the transistor 12. In one embodiment, the control switch 45 includes a transistor 45, a first input terminal of the control switch 45 is a source of the transistor 45, a second input terminal of the control switch 45 is a gate of the transistor 45, and an output terminal of the control switch is a drain of the transistor 45. When the control switch 45 receives the second control signal S _ C2 from the operational amplifier 44, the second control signal S _ C2 adjusts the gate voltage of the transistor 45 such that the control switch 45 turns on the injection current or turns off the injection current. More specifically, when the second control signal S _ C2 turns on the control switch 45, the transistor 45 is turned on, and the gate voltage of the transistor 12 is pulled up, so that the input current I of the voltage regulator 10 is increased_INWill be adjusted accordingly when the output voltage V is_OUTThe smaller the input current I allowed_INThe smaller the maximum value of the voltage regulator 10, the less power consumption of the voltage regulator 10 and the better protection of the PMOS transistor 12 from damage can be achieved.

Power consumption P of PMOS transistor 12_LOSSAnd an input current I_INInput voltage V_INAnd an output voltage V_OUTThe relationship between them can be expressed by the mathematical formula (3):

P_LOSS＝I_IN*(V_IN–V_OUT) (3)

in summary, when the input current I is reached_INWhen the current is limited, the load impedance of the voltage regulator 10 becomes smaller, and the voltage regulator 10 outputs the voltage V_OUTWill also gradually become smaller, resulting in power consumption P of PMOS transistor 12_LOSSGradually becoming larger.

Referring to fig. 5, fig. 5 is a signal simulation diagram of an embodiment of a current limiter according to the present invention. As can be seen from fig. 3, the variable current source 43 regulates the output current I when the current limiting mechanism is activated_MODAnd the output current I of the sensing current source 41_SENThe sum of (A) and (B) is kept constantMeasuring the output voltage V of the current source 41_SENMaintaining a constant value, i.e. the second reference voltage V_OCREF. When the partial pressure V is_FBIn the fall, the variable current source 43 regulates the output current I_MODWill gradually rise to sense the output current I of the current source 41_SENWill gradually decrease along with the output current I of the sensing current source 41_SENIs reduced, the input current I of the voltage regulator 10_INAnd also decreases.

Referring to fig. 6A and 6B, fig. 6A is a graph showing a relationship between an output voltage and an output current when the current limiting mechanism is activated in fig. 3, and fig. 6B is a graph showing a relationship between an output voltage and a power consumption of a transistor when the current limiting mechanism is activated in fig. 3. It can be seen from fig. 6A that the current limiter 40 acts as an output voltage V through the embodiment of the present invention_OUTWhen the voltage becomes smaller than a critical value, the voltage is divided by V_FBAlso gradually becomes smaller and approaches the first reference voltage V_MREFInput current I of voltage regulator 10_INMaximum value of (1)_{OUT_CL}It also begins to get smaller. As can be seen from FIG. 6B, when the input current I of the voltage regulator 10 is applied_INWhen reduced, the power consumption P of transistor 12_LOSSAnd is reduced accordingly.

While the invention has been described in terms of preferred embodiments, it will be understood by those skilled in the art that the examples are intended in a descriptive sense only and not for purposes of limitation. It should be noted that equivalent variations and substitutions to those of the embodiments are intended to be included within the scope of the present invention. Therefore, the protection scope of the present invention is subject to the scope defined by the claims.

Claims

1. A current limiter for a voltage regulator, the current limiter comprising:

the sensing current source is electrically connected with the voltage regulator and used for generating an induction current according to the input current of the voltage regulator;

the current limiting and adjusting unit is electrically connected to the voltage regulator and used for generating a first control signal according to the output voltage of the voltage regulator and a first reference voltage;

the variable current source is connected in parallel with the sensing current source, is electrically connected with the current limiting and adjusting unit and is used for adjusting the generated current according to the first control signal;

the operational amplifier is electrically connected to the sensing current source and used for generating a second control signal according to a sensing voltage and a second reference voltage generated by the currents generated by the sensing current source and the variable current source; and

a control switch electrically connected to the voltage regulator and the operational amplifier for supplying a gate of a transistor of the control switch according to the regulated second control signal;

when the divided voltage of the output voltage of the voltage regulator gradually decreases, the variable current source gradually adjusts and increases the current generated by the variable current source according to the first control signal.

2. The current limiter of claim 1, wherein the sensing current source has an input electrically connected to an input voltage of the voltage regulator and an output, and further comprising:

the output end is grounded through the resistor;

the magnitude of the output current of the sensing current source is proportional to the input current of the voltage regulator, and the voltage across the resistor is the output voltage of the sensing current source.

3. The current limiter of claim 1, wherein the current limiting adjustment unit has a first input terminal, a second input terminal and an output terminal, the first input terminal is connected to ground through a first voltage dividing resistor of the voltage regulator, the second input terminal is connected to the first reference voltage, and the output terminal is electrically connected to the variable current source and outputs the first control signal to the variable current source through the output terminal.

4. The current limiter of claim 2, wherein the variable current source has a first input terminal, a second input terminal and an output terminal, the first input terminal is electrically connected to the input terminal of the sensing current source, the output terminal is electrically connected to the output terminal of the sensing current source, and the second input terminal is electrically connected to the current limiting adjustment unit.

5. The current limiter of claim 2, wherein the operational amplifier has a negative phase input terminal electrically connected to the output terminals of the sensing current source and the variable current source, a positive phase input terminal connected to the second reference voltage, and an output terminal electrically connected to the control switch, and outputs the second control signal to the control switch through the output terminal.

6. The current limiter of claim 2, wherein the control switch has a first input electrically connected to an input voltage of the voltage regulator, a second input electrically connected to the operational amplifier, and an output connected to a gate of a transistor of the voltage regulator.

7. The current limiter of claim 6, wherein the control switch comprises:

the first input end of the control switch is a source electrode of the transistor, the second input end of the control switch is a grid electrode of the transistor, and the output end of the control switch is a drain electrode of the transistor.

8. The current limiter of claim 1, wherein the sensing current source is a current mirror or a current amplifier.

9. The current limiter of claim 1, wherein the current limiting adjustment unit is a differential amplifier.